i wonder if this piston was not in the cylinder all the way when the crank was put down on it. Maybe the weight of the crank caused the piston which was sitting tilted in the bore to hit on the edge of the cylinder and bend some before going into the hole. but i dont think your piston was installed improperly.
only other thing i could think of is a piece of the end of the ring broke off and lodged in there.
whats the cylinder wall look like?